Semi-transparent Colors in R: Color Image as an Example

There are many graphical functions offering the availability of the parameter alpha which is usually used to specify semi-transparent colors, however, such kind of colors can only be displayed in certain devices, as stated in the help of rgb():

Semi-transparent colors (0 < alpha < 1) are supported only on some devices: at the time of writing only on the pdf and (on MacOS X) quartz devices as well as several third-party devices such as those in packages Cairo, cairoDevice, JavaGD and RSvgDevice.

Here is an example illustrating semi-transparent colors in a pdf() device:

(tmp = file.path(tempdir(), "alpha.pdf"))
pdf(tmp) # open a pdf device in the temp dir
plot(rnorm(100), ylim = c(-3, 3), xlab = "x", ylab = "y",
   main = "Add a semi-transparent color image to a graph")
image(x = seq(1, 100, length = 20), y = seq(-3, 3,
   length = 20), z = matrix(rnorm(400), 20), col = heat.colors(20,
   alpha = 0.5), add = T) # here we set alpha = 0.5
dev.off()

You may try other devices such as postscript(), jpeg(), or bmp(), etc, and you will find alpha is not supported there.